Digestion and metabolism of low and high residual feed intake Nellore bulls

Understanding the reasons why animals of similar performances have different feed requirements is important to increase profits for cattle producers and to decrease the environmental footprint of beef cattle production. This study was carried out aiming to identify the associations between residual feed intake (RFI) and animal performance, nutrient digestibility, and blood metabolites related to energy balance of young Nellore bulls during the finishing period. Animals previously classified as low (n = 13) and high RFI (n = 12), with average initial body weight of 398 kg and age of 503 days were used. Cattle were fed a high energy diet and were slaughtered when rib fat thickness measured by ultrasound between the 12th and 13th ribs reached the minimum of 4 mm. A completely randomized design was adopted, being data analyzed with a mixed model that included the random effect of slaughter group, the fixed effect of RFI class, and linear effect of the covariate feedlot time. No differences were found (p > 0.10) between RFI classes for performance, dry matter, and nutrients intake. However, dry (p = 0.0911) and organic matter (p = 0.0876) digestibility tended to be lower, and digestibility of neutral detergent fiber corrected for ash and protein (p = 0.0017), and total digestible nutrients (p = 0.0657) were lower for high RFI animals, indicating lesser capacity of food utilization. Difference between low and high RFI animals was also found for blood cortisol at the end of the trial (p = 0.0044), having low RFI animals lower cortisol concentrations. Differences in the ability to digest food can affect the efficiency of transforming feed into meat by Nellore cattle.